1. Field of the Invention
This invention is related to an on/off reversible adhesive and a method for fabricating same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within parentheses, e.g., (x). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The mechanism of adhesion in the gecko has been of scientific interest since Aristotle observed a gecko running up and down a tree (1). Recently there has been an intensifying scientific investigation into the fundamental physics of the adhesive, isolating van der Waals as the primary interaction (2, 3), with additional evidence that humidity contributes substantially to the adhesion (4, 5). Regardless of the fundamental interactions at the atomic scale, there is clearly a great deal of function contained in the hierarchical structure of the gecko's tarsus (6-12). The structure consists of 200 nm wide spatular termini at the ends of ˜100 μm long, ˜5 μm diameter setae, which extend from a compliant tarsal pad cushioned by blood sinuses (10-13). This fine splitting of the contacts into sub-micron elements on slender setae increases the van der Waals interaction with the surface, and allows easy deformation of the attachment system to accommodate rough surfaces (15). In addition, the gecko can increase the adhesion strength by prestressing (squeezing) opposite toes through muscular activation; this transforms some component of the normal force into a shear force, thus enhancing adhesion. There is evidence that the hierarchical structure may serve another purpose—to reduce adhesion, when needed (2, 14, 15).
Just as interesting as the gecko adhesion mechanism is the mechanism by which the gecko is able to release a surface. The specialized shape of the gecko's setae and spatulae allow the gecko to control adhesion through macroscopic movements of its toes. By hyperextension of its toe, the gecko peels its adhesive pads off the surface. This, in itself, aids in the detachment of the foot, just as peeling a piece of tape off of a surface is easier than trying to pull it off all at once. It has been shown theoretically (16) and experimentally (3) that the magnitude of setal adhesion is greatly dependent on the angle of applied pull-off force. Thus, when the gecko switches from a squeezing (adhesive) state to a peeling state, the adhesive force goes from a maximum to a minimum. This can also be seen at the nanoscale, as the applied force on the spatulae changes the interactions from a frictional regime to a peeling regime, greatly reducing the adhesion (6). Therefore, the design of its hierarchical attachment structure allows the gecko to control the adhesion from the macro-scale to nano-scale through macroscopic muscle movements.
Previously, a bio-inspired synthetic system enhancing adhesion utilizing a hierarchical structure was fabricated and tested (17). Prior work has focused on mimicking the terminal bristle component of the adhesive, by fabricating arrays of polymeric nanorods on a solid substrate (3, 20, 21). While individual nanorods demonstrated expected amounts of adhesion, larger arrays failed to produce larger amounts of adhesion, unless removed from the substrate and placed on a compliant backing (20), showing the need for multiscale compliance. In addition, unlike the gecko, the system did not provide a mechanism for decreasing adhesion. This attribute is critical to any application of such a biomimetic system.